Flow control method and device

ABSTRACT

An arterio-venous graft ( 16 ) is provided with a constriction device ( 20 ) near its arterial end. The constriction device ( 20 ) is used to reduce the flow through the AV graft under normal conditions and to relieve the constriction when high flow through the AV graft is required, such as for vascular access for hemodialysis.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 11/586,886, filed on Oct. 26, 2006, which is acontinuation application of U.S. patent application Ser. No. 10/031,469,filed on May 29, 2002, now U.S. Pat. No. 7,128,750, which is a section371 of Application No. PCT/EP00/06907, filed on Jul. 19, 2000, whichclaims the benefit of European Application No. 99305689.4, filed on Jul.19, 1999. The entire contents of each of these applications are herebyincorporated by reference herein.

FIELD

This invention relates to a method of flow control of a bodily vessel,for example for use in an arterio-venous graft, hereinafter referred toas an AV graft. The invention also relates to a device for controllingflow in a bodily vessel, such as an AV graft, and a combination of sucha device and a graft.

BACKGROUND

Patients with kidney disease, particularly those with end stage renaldisease (ESRD), require hemodialysis in order to remove metabolites andthe like from their blood stream. This can be a very time-consumingprocess, but the time can be lessened by providing a large blood flow tothe hemodialysis machine. Even though this is done, hemodialysis canstill take about four hours and is needed about three times a week.

In order to provide high blood flow to and from the hemodialysismachine, vascular access with high blood flow is needed. One method ofproviding this is illustrated in FIG. 1. An artery 10 and a vein 12 arelocated in the arm 14 of the patient. A vessel 16, known as an AV graftor shunt, is grafted to connect the artery 10 and the vein 12. As the AVgraft 16 is a direct connection between the artery 10 and the vein 12and has a relatively large cross-sectional area, a high flow through itoccurs. The direction of flow is indicated by the arrows in FIG. 1.Catheters (not shown) can be connected to the AV graft 16, whenhemodialysis is required. The catheters can tap into the high flowthrough the AV graft 16 to provide a high flow to and from thehemodialysis machine.

However, there are also considerable problems with this technique. Oneof these, illustrated in FIG. 2, is that stenosis 18 occurs at theoutflow tract where the AV graft 16 is connected to the vein 12, that isat the venous anastomosis side of the graft. The stenosis 18 is anunnatural narrowing of the vessel, and if unopened by angioplasty, thestenosis 18 progresses until the vein 12 is completely blocked. Thestenosis 18 is due to neo-intimal hyperplasia, that is the response ofthe vessel 16 to the abnormal conditions.

Various mechanisms are considered as possibly contributing to thedevelopment of the stenosis 18. The flow through the vein 12 istypically 10 to 20 times higher than normal. This leads to turbulenceand flow separation such that the flow is not smooth or laminar, and thestenosis 18 develops as a result. Another factor is that the vein 12 isexposed to a higher blood pressure than normal, because it is directlyconnected to the artery 10. The blood pressure in an artery 10 istypically 100 mm Hg, whereas the blood pressure in a vein 12 istypically 5 mm Hg. The vein 12 tends to arterialize in response to this,for example by thickening of the vein wall and this may contribute tothe stenosis 18. A further possible factor is that, in the presence ofthe graft, the flow in the vein 12 is pulsatile. There is a significantcompliance mismatch between the AV graft 16, which, if synthetic, isquasi-rigid, and the vein 12 which is compliant. The pulsatile flowproduces an oscillating stress concentration at the junction, i.e.suture line, between the AV graft 16 and the vein 12. Although thesuture usually does not fail, the stenosis 18 may be in response to theoscillating stress concentrated at the junction.

This is a considerable problem. In 90% of AV grafts (e.g., AV graft 16),stenosis 18 develops at the venous anastomosis side. AV graft survivalis around only 1.5 years. Conventionally, alleviation of this problemrequires surgery, such as angioplasty to remove the stenosis 18 orsurgery to implant a new AV graft in a different limb of the patient.

A further problem is that the AV graft 16 effectively provides a shortcircuit between the artery 10 and the vein 12 and the high flow throughthe AV graft 16 requires a huge additional cardiac output. Normalcardiac output is typically 5 liters per minute, but with the AV graft16 in place this can increase to 7 liters per minute. This largeadditional cardiac output can be very problematic indeed, and can resultin fatal cardiac failure for about 5% of AV graft patients.

SUMMARY

According to the present invention there is provided a method of flowcontrol in an AV graft or an AV fistula used for vascular access for anextracorporeal circuit, said method comprising the steps of:

(a) applying partial constriction to a vessel to provide a reduced flowthrough the AV graft or the AV fistula, when flow through theextracorporeal circuit is not occurring; and

(b) changing the degree of constriction, to modify the flow through theAV graft or the AV fistula, when flow through the extracorporeal circuitis to occur.

Applying partial constriction can reduce or eliminate turbulence, andlower the blood pressure in the vein. The constriction can also act as astrong wave reflector to reduce or eliminate the pulsatile flow at thevenous anastomosis. All of these can alleviate stenosis, prolong thelife of the AV graft or the AV fistula and reduce the necessary cardiacoutput. Changing the degree of constriction when the flow through theextracorporeal circuit is to occur enables a high flow to be providedfor vascular access.

The constriction of the vessel is only partial, preferably to maintain areduced but significant residual flow through the AV graft to avoidthrombosis, and to keep the vein matured and able to handle the highflow when necessary.

Preferably the constriction is applied over an elongate portion of thevessel. This enables the flow control to be achieved by viscousdissipation in favor of turbulent dissipation.

Preferably the constriction is applied at a plurality of positions alongthe vessel and/or the profile of the constriction is controlled alongits length. This enables turbulence caused by the constriction to beminimized.

Preferably the constriction reduces the cross-sectional area of thelumen of the vessel, but maintains the length of the perimeter thereof,again to favor viscous dissipation.

Preferably, when applying the constriction to the vessel, the flow atthe venous anastomosis of the AV graft or the AV fistula is monitored sothat when constricted, the flow is maintained at a level below the onsetof turbulence.

Preferably the vessel is an AV graft.

Preferably the constricting step comprises constricting the AV graft atits arterial end. This enables any turbulence caused by the constrictionto subside before the blood flow reaches the venous anastomosis.

The invention provides a device for controlling flow in an AV graft oran AV fistula used for vascular access for an extracorporeal circuit,the device comprising:

a) means for applying partial constriction to a vessel, to provide areduced flow through the AV graft or the AV fistula, when flow throughthe extracorporeal circuit is to occur; and

b) means for changing the degree of constriction, to modify the flowthrough the AV graft or the AV fistula, when flow through theextracorporeal circuit is to occur:

The invention also provides a device, for controlling flow in a bodilyvessel, the device comprising an actuator for releasably constrictingthe bodily vessel, and a rotatable member for driving the actuator.

Preferably the rotatable member comprises a drive shaft of a motor orcomprises a rotor rotatable by an externally applied magnetic field.

Preferably the motor is an electrical micromotor.

The invention also provides a device, for controlling flow in a bodilyvessel, the device comprising a deformable member which is reversiblydeformable by a change in temperature or magnetic field, and an actuatoracted on by the deformable member for releasably constricting the bodilyvessel, wherein the deformable member is deformable between a firststate in which the actuator applies constriction to the bodily vessel,and a second state in which the actuator reduces the constriction of thebodily vessel.

Preferably the thermally deformable member comprises a shape-memorymaterial or a liquid filled capsule.

Preferably the device of the invention further comprises an antenna forreceiving signals for controlling the actuator. This avoids the need foraccess to the device through the skin and the potential risk ofinfection.

Preferably the device further comprises a converter for converting radiofrequency energy received by the antenna into energy for powering thedevice to operate the actuator. This has the advantage of avoiding theneed for an internal power source, such as a battery, in the device, andradio frequency activated devices are NMR-proof.

The invention further provides a device, for controlling flow in abodily vessel, the device comprising an actuator for releasablyconstricting the bodily vessel, wherein the actuator comprises a cliphaving two constriction portions with an adjustable separationtherebetween for accommodating the bodily vessel and a control portionfor releasably holding the two constriction portions such that theseparation is held at least one predetermined amount.

Preferably the constriction portions are integrally formed as one memberthat makes the device simple and cheap to fabricate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of a human lower arm, illustrating aconventional AV graft in situ;

FIG. 2 is a close-up view of the venous anastomosis of FIG. 1,illustrating a problem associated with the AV graft of FIG. 1;

FIG. 3 is a schematic view of a human lower arm, illustrating anarrangement according to the present invention;

FIGS. 4( a) and 4(b) are schematic cross-sectional views of a firstembodiment of apparatus according to the present invention, shownapplied to an AV graft;

FIG. 4( c) is a plan view of a deflectable membrane of an embodiment ofthe invention;

FIG. 5 shows a second embodiment of an apparatus according to thepresent invention;

FIGS. 6( a) and 6(b) show a third embodiment of an apparatus accordingto the invention in cross-section and plan view, respectively;

FIGS. 7 and 8 show cross-sectional views of fourth and fifth embodimentsof apparatus according to the invention;

FIGS. 9, 10 and 11 are explanatory diagrams for illustrating furtheraspects of the present invention;

FIG. 12 illustrates schematically an embodiment of the inventionincorporating an electromagnetic flow measurement system; and

FIG. 13 illustrates an application of the invention to a Blalock-Taussigshunt.

DETAILED DESCRIPTION

FIG. 3 shows an arrangement according to the present invention, withcorresponding parts labeled the same as in FIG. 1. The AV graft 16 maybe an artificial vessel, for example, made of PTFE or GORE-TEX, othersynthetic material, or the AV graft 16 may be an autologous graft. Asillustrated in FIG. 3, the AV graft 16 is connected to an artery 10 anda vein 12 in the arm 14 of a patient. However, the AV graft 16 may, ofcourse, be located in other parts of the body, for example the leg,groin or neck.

A device 20 is provided for controlling blood flow in the AV graft 16.During the normal activities of the patient, the device 20 is used toconstrict the AV graft 16 such that there is a reduced or residual flowtherethrough. When flow through an extracorporeal circuit, such as ahemodialysis machine, is required, the degree of constriction isreduced, partially or fully, so that there is an increased, high flowthrough the AV graft 16. Catheters (not shown) can tap into the highflow in the AV graft 16 to provide high flow to and from a hemodialysismachine. The catheters may be upstream or downstream of the device 20 ormay be provided on opposite sides of the device 20. A single catheterwith a double lumen may also be used for flow to and from the AV graft16.

As illustrated in FIG. 3, the constriction device 20 is used toconstrict the AV graft 16 at its upstream end, in the vicinity of itsconnection with the artery 10. Preferably the constriction device 20 iswholly implanted within the patient and an external controller 22 isused telemetrically to control the constriction device 20.

When high flow through the AV graft 16 is no longer required, theconstriction device 20 is used to re-apply constriction to reduce bloodflow. A turbulence measuring device 24, 26 may be used to monitorturbulence in the vicinity of the venous anastomosis while the flowthrough the AV graft 16 is being reduced. As the degree of constrictionis increased, the flow rate reduces such that a level will be reached atwhich turbulent flow substantially ceases to be detected by theturbulence measuring device. When this occurs, further change inconstriction can be stopped and the flow maintained at that level belowthe onset of turbulence. Alternatively, the constriction may beincreased until the turbulence has been diminished to a predeterminedlevel, but not totally abolished. Preferably this diminished turbulenceintensity is below the level at which stenosis 18 may occur, but theflow rate is still sufficient to keep the vein 12 matured. In this wayan optimal quiescent flow can be established in the vicinity of thevenous anastomosis side of the AV graft 16.

The turbulence measuring device 24, 26 can be a conventional Dopplerdevice or a phonoangiographer and may advantageously be connected to thecontroller 22 or constriction device 20 automatically to controladjustment of the flow rate, or this may be done manually.

Further features of the method of the present invention will be apparentfrom the following description of devices according to the invention.

FIGS. 4( a) and 4(b) are longitudinal and transverse cross-sections,respectively, of a constriction device 20 and a control device 22. Thecontrol device 22 has an antenna 30 for transmitting signals to anantenna 32 provided on the constriction device 20. The antennae 30, 32are electromagnetically coupled to each other, but are of course onopposite sides of the skin (not shown) of the patient. A receiver 34connected to antenna 32 sends electrical power to a motor 36 in responseto the transmitted signal.

The constriction device 20 may contain an internal power source, such asa battery, which is controlled by the receiver 34 to deliver electricalpower to the motor 36. Alternatively, the receiver 34 may comprise aradio frequency to DC converter and modulator, in which case radiofrequency signals emitted by the antenna 30 are picked up by the antenna32 and these signals are converted by the receiver 34 into electricalpower to drive the motor 36, rather than the signals being used tocontrol an internal power source of the device, thereby eliminating theneed for batteries in the device which would need to be replacedperiodically.

The motor 36 is a miniature motor, also known as a micro-machine, andwhen provided with electrical power it can be used to rotate a driveshaft 38 in either direction, or in one direction only, provided thatthe actuator performs a periodic displacement even if the micromotorshaft 38 always turns in the same direction. The dimensions of themicromotor 36 are sufficiently small to enable it to be encapsulated inan implantable enclosure, for example the motor may be 2 mm thick and 15mm long. A rotary to linear transmission 40 converts the rotation of thedrive shaft 38 into linear motion of an actuator comprising members 42,44 and 46. Members 42 and 44 are rods or bars and member 46 is, forexample, a fine titanium membrane that is in contact with the AV graft16 or presses upon the AV graft 16 through an intermediate material.

As shown in FIGS. 4( a) and 4(b), the actuator 42, 44, 46 isconstricting the AV graft 16, such that the cross-sectional area of itslumen 48 is reduced. By sending appropriate signals, and through actionof the motor 36, the constriction can be relieved by motion of theactuator, when high flow is required, and the position of the membrane46 in this high flow state is indicated by the dashed line 50.

The constriction device 20 is encapsulated in an enclosure 52, such as atitanium or ceramic box, through which the AV graft 16 can pass, or intowhich the AV graft 16 can be slotted sideways. The antenna 32 asillustrated in FIGS. 4( a) and 4(b) is located outside the enclosure 52so that it is not screened by the enclosure and to enable the antenna 32to be placed under the skin for optimal RF wave reception. Thisarrangement of having the antenna 32 external to and optionally remotefrom the enclosure 52 can be advantageous for cases in which theconstriction device 20 is implanted deep within the body and the RFwaves from the external control unit have a maximum penetration depth of2 to 4 cm. Alternatively, for situations in which the constrictiondevice 20 can be implanted just under the skin or not too deep, theantenna 32 can be internal, i.e. encapsulated within the enclosure 52 ofthe constriction device 20. In this alternative embodiment, theenclosure 52 or at least part of the enclosure 52 is non-metallic, forexample ceramic or plastic to avoid screening of the RF waves. Havingthe antenna 32 internal or integral to the enclosure 52 of theconstriction device 20 is advantageous in simplifying the implantationof the device within the body.

The device may optionally include a sensor, not shown, such as a sensorfor measuring the position of the actuator or for counting the number ofrevolutions of the drive shaft 38. Sensors for measuring flow,turbulence or pressure may also be included. Information from thesensor(s) can then be transmitted from the constriction device 20 to thecontrol device 22 via the antennas 30, 32, so that the controller 22 cancontrol the constriction more precisely.

FIG. 5 illustrates an alternative constriction device 20 in the form ofa clip. The actuator of the device comprises a pair of constrictionportions 60, 62 separated by a gap through which the AV graft 16 passes.The separation between the constriction portions 60, 62 can be reducedby applying pressure to the skin 64 of the patient to constrict the AVgraft 16. A control portion 66 comprises a series of grooves or notchesengageable by an insertion portion 68 of the constriction portion 60.Pressure applied to the skin 64 moves the insertion portion 68 from theposition shown in FIG. 5 into successively lower notches. When therequired level of constriction is achieved, the engagement of theinsertion portion 68 in the particular notch of the control portion 66maintains that level of constriction.

A pressing device 70 may be used for this process and may comprise asensor that detects the motion of the insertion portion 68 from onenotch to the next so that the position of the constriction portions isknown and an optimal level of constriction applied.

When high flow through the AV graft 16 is required, the constriction canbe reduced by again applying pressure to the skin of the patient, butthis time by pressing on a release portion 72. This splays the controlportion 66 so that the insertion 68 disengages from the notches and theopening between the constriction portions 60, 62 increases.

As shown in FIG. 5, the constriction device 20 is formed from a singlepiece, such as by molding it from a biologically compatible plasticsmaterial. This makes it very simple and cheap to fabricate.

Another embodiment of the constriction device is shown in FIGS. 6( a)and 6(b). It comprises an actuator plate 80, within an enclosure 82, forsqueezing on the AV graft 16. A rotor 84 is screwed onto a threadedshaft 86. The rotor 84 comprises a series of magnetic north and southpoles alternating around the shaft 86. The rotor 84 can comprise anysuitable magnetic material, such as ferrite.

Application of an alternating or rotating magnetic field from outsidethe patient can cause the rotor 84 to revolve about the axis of theshaft 86. The threaded engagement between the rotor 84 and the shaft 86causes the rotor 84 to translate in the axial direction of the shaft 86,the direction of translation depending on the sense of rotation of therotor 84. In this way the externally magnetic field can be used to movethe rotor 84 along the shaft 86 to urge the actuator plate 80 againstthe AV graft 16 to apply constriction thereto, or to release pressurefrom the actuator plate 80 and reduce the constriction when high flowthrough the AV graft 16 is required.

FIGS. 7 and 8 show two further embodiments of the constriction device 20of the invention which both operate thermally. Each device has anactuator comprising a movable member 90 and a flexible membrane 92 forconstricting an AV graft 16, the device being housed in an enclosure 94.

In the embodiment of FIG. 7, the actuator member 90 is connected to asheet 96 made of a heat-deformable material. This is shown in its normalstate at body temperature whereby the AV graft 16 is constricted toreduce the quiescent flow therethrough. On raising the temperature ofthe sheet 96 it deforms-into the shape indicated by the dashed line 98thereby pulling on the actuator 90, 92 to reduce the constriction on theAV graft 16. The material of the sheet 96 may be a shape-memorymaterial, such as a so-called smart metal, or it could be a bi-metallicstrip or any other suitable material that deflects on changingtemperature, or a shape memory material that is magnetically activated.

In the device of FIG. 8, the actuator member 90 is connected to adeformable membrane 100 which defines one surface of a liquid filledcapsule 102 containing a liquid with a low boiling point, such as justabove body temperature, for example around 39 degrees Celsius (C). Undernormal conditions the capsule 102 contains liquid and the actuator 90,92 squeezes the AV graph 16 to reduce blood flow. On increasing thetemperature of the substance in the capsule 102 above its boiling point,at least some of the liquid vaporizes which results in an overallincrease in volume of the contents of the capsule 102. This expansiondeflects the membrane 100 and a force is transmitted via the member 90to lift the flexible membrane 92 to relieve the constriction of the AVgraph 16. The position of the deformable membrane 100 when in this stateis indicated by the dashed line 104.

The devices 20 shown in FIGS. 7 and 8 may be provided with an optionalheater 106, such as an electrical resistance. When it is desired toincrease the blood flow through the AV graft 16, electric current ispassed through the heater 106 to raise the temperature of the sheet 96or liquid filled capsule 102 to move the actuator as described above.The electrical current may be provided by a battery associated with thedevice and controlled by signals from an external controller asdescribed with reference to FIGS. 4( a) and 4(b), or the electricalcurrent may be provided by a radio frequency converter which convertsradio frequency radiation into electrical power, without the need for aninternal battery, as also described with reference to FIGS. 4( a) and4(b). Alternatively, the increase in temperature necessary to change thestate of the thermal device may be provided by an external heat source.This eliminates the need for the heater 106. The external heat sourcemay take the form of, for example, an infrared lamp directed onto theskin in the vicinity of the device 20. The heater 106 could also be anantenna which heats up when an appropriate electromagnetic field isapplied.

When high flow through the AV graft 16 is no longer required, such aswhen hemodialysis has been completed, power to the heater 106 is cutoff, or the external heat source removed. The sheet 96 or fluid filledcapsule 102 cools back to normal body temperature and returns to theconfigurations shown in FIGS. 7 and 8 in which the actuator 90, 92 issqueezing the AV graft 16.

All of the above described constriction devices are intended to bewholly implantable within the patient. The enclosures 52, 82, 94comprise a titanium, ceramic or plastic box and the dimensions of thesides in transverse cross-section may be in the region of 10 to 30 mm,the unconstricted diameter of an AV graft (e.g., AV graft 16) beingtypically 5 to 8 mm. The flexible membrane 46, 92, in contact with theAV graft 16 may be a very thin (i.e. 20 to 60 micrometers thick)titanium sheet or a thicker titanium membrane preferably withappropriate corrugations 47 to facilitate deflection, as shown in planview in FIG. 4( c). The corrugations 47 can be seen in cross-section inFIGS. 4( a), 4(b), 7 and 8. The region surrounding the AV graft 16, butwithin the respective enclosure, such as the region 110 shown in FIGS. 7and 8 may contain a deformable, but incompressible, material such as gelto control the constriction of the AV graft 16.

FIG. 9 shows schematically a constriction, such as in an AV graft 16.The normal diameter of the vessel is D, the constricted diameter is d,and the constriction is applied over a length L. It is preferred thatthe method and devices of the present invention apply the constrictionover an elongate portion of the AV graft 16, for example as shown inFIG. 4( a). Preferably the length L is at least twice the originaldiameter D, and L may even be five to ten or more times the diameter D.The reasons for this are as follows. For a given flow rate Q through theAV graft 16, the viscous losses are proportional to LQ, whereas theturbulent losses are proportional to [(D/d)²−1]²Q². These two lossescontribute to the overall dissipation caused by the constriction whichresults in the pressure drop and reduced flow rate. An acute localizedconstriction produces much turbulence which can cause thrombosis orunwanted stenosis (e.g., stenosis 18) downstream at the venousanastomosis, or in the AV graft 16 itself if it is made of livingtissue. The same overall flow reduction can be achieved by increasingthe length of the constriction to increase viscous loss, but reducingturbulent loss.

One way to increase the length of the constriction is to providemultiple constriction devices in series along the AV graft 16. Anothermethod is to provide a single elongate actuator within the device ormultiple actuators disposed along the length of the device.

FIG. 10 illustrates a further technique for reducing turbulence causedby the constriction, namely by controlling the profile of theconstriction such that abrupt transitions in diameter are avoided. Theprofile of the constriction can be controlled by providing a pluralityof actuators 120, each of which squeezes the AV graft 16 by a controlledamount. The actuators 120 may all be provided within a singleconstriction device, or each actuator 120 may be provided in arespective constriction device disposed in series along the AV graft 16.Alternatively, a single actuator of a predetermined profile may be usedto cause a desired constriction profile.

A further technique for favoring viscous dissipation over turbulentdissipation is illustrated with reference to FIG. 11. A transversecross-section of the unconstricted AV graft 16 is approximately circularas shown in the center of FIG. 11. Applying an isotopic force around theperiphery to squeeze the vessel approximately equally in all directionswould tend to reduce the cross-section of the vessel to be a circle ofsmaller diameter. However, viscous losses are related to the area of thewall of the vessel and hence to the perimeter of the cross-section. Bysqueezing the AV graft 16 unequally in different directions, theperimeter of the lumen can be maintained substantially constant inlength while reducing its cross-sectional area. Various exemplaryresulting shapes are shown in FIG. 11. The arrows illustrate thedirections and points of application of the squeezing force. The devicesaccording to the invention can achieve constrictions of these shapes bya variety of ways, such as having ridged actuators, or a plurality ofactuators applying pressure in different directions or surrounding theAV graft 16 by a gel to control the shape of the deformation.

A further feature of the invention is to adhere the outer surface of theAV graft 16 to the actuator (e.g., actuator 42, 44 or 46) using a glue.According to Bernoulli's equation, p+¹/2ρν² is constant, where p ispressure, ρ is viscosity and ν is flow velocity. At a constriction, theflow velocity increases to maintain throughput. At sufficiently highvelocity, the pressure given by Bernoulli's equation can become lowerthan the external pressure on the vessel or even become negative. Thus,at a constriction it is possible for collapse of the vessel to occurbecause the reduced pressure sucks the walls inwards. The flow of coursethen stops and the vessel recovers, but vessel collapse is problematicand results in erratic flow conditions. Gluing the wall of the AV graft(e.g., AV graft 16) to the actuator (e.g., actuator 42, 44, or 46)prevents collapse by maintaining a minimum diameter of the AV graft(e.g., AV graft 16), even when constricted. Collapse of the AV graft(e.g., AV graft 16) may also be prevented if the constriction isappropriately shaped, as shown in some of the examples in FIG. 11, toresist further buckling under reduced pressure.

As previously mentioned, in one arrangement catheters for extracorporealflow to and from the AV graft 16 may be provided on opposite sides ofthe device 20. In this case it can be beneficial to increaseconstriction of the graft during e.g. hemodialysis in order to augmentflow through the extracorporeal machine. For the rest of the time, theconstriction is still partially applied to alleviate the problems, suchas caused by turbulence, whilst keeping the vein (e.g., vein 12)matured.

The method and device of the invention can also be used with AVfistulas, in which case the flow control device is placed on the arteryor the vein, just proximal or distal to the fistula, respectively.

A further preferred aspect of the invention, which can be used with anyof the above-described embodiments, is to incorporate a flow-measuringdevice into the variable flow control device 20. FIG. 3 illustrated anexternal flow or turbulence measuring device 24, 26, however, accordingto the present further embodiment, the implanted device incorporatesflow-measuring apparatus. The flow measured by the device may becommunicated, for example, via an antenna 32, to an external device togive a reading of the flow passing through the AV graft (e.g., AV graft16). Alternatively, or in addition, the flow measurement may be usedinternally within the implanted device to control the constrictionapplied, using a feed-back loop, to regulate the flow.

Examples of two technologies that can be used in embodiments of the flowcontrol device for measuring flow are described below.

(1) Ultrasonic Flow Measurements

A piezo-element emits ultrasound, which is reflected by the flowingblood, the reflected signals being slightly changed in frequency throughthe Doppler effect, thereby carrying information on velocity which isdetected. Referring to FIG. 4( a) by way of illustrative example, theinformation on velocity is transmitted from the implanted device 20 viathe antenna 32 to the external antenna 30 and is then received anddisplayed by the external control unit 22.

(2) Electromagnetic Flow Sensor

The flow meter according to this embodiment works on the principle ofFaraday's Law of Induction, which states that if a conductor is movedwithin a magnetic field, a voltage is induced at right angles to thedirection of movement in that conductor and at right angles to themagnetic field. The voltage generated is proportional to the averagevelocity of the moving conductor. The voltage signal U is proportionalto the product vDB, where U=voltage across the channel, v=conductoraverage velocity, D=distance between the electrodes and B=magnetic fluxdensity.

An example of this embodiment is illustrated in FIG. 12. The blood inthe vessel 16 acts as the moving conductor. A magnetic field B can beapplied by an external magnet. Preferably the magnetic field coming fromthe external antenna 30 is used. This is advantageous because iteliminates the need to install magnets or other means of imposing amagnetic field. Preferably the magnetic field is alternating, in whichcase a different frequency of B is used other than the one used for thecontrol of the flow control device 20. Thus the external control unit 22emits one frequency, for example, for the telemetric control of a motor36 and for power generation, and another frequency for creating themagnetic field B required for the flow measurement.

The voltage measuring electrodes measuring 120 are placed perpendicularto B and v, and with a precisely known separation D. The EMF generatedbetween the electrodes 120 is sensed by a voltage measuring device 122.For improved measurement sensitivity, the voltage measuring device 122is tuned to the frequency of the externally applied magnetic field B.The electrodes 120 can be encapsulated either in the main box of thedevice 20 or in an auxiliary chamber next to the main box.

All of the preceding methods and devices according to the invention havebeen described in terms of application to an AV graft. However, asmentioned in the introduction, they can also be applied to the variableflow control of other bodily vessels, by which is meant a generallytubular structure that transports material in the body, such as a bloodvessel, lymph vessel, vessel in the digestive tract, vessel in theurinary tract, vessel in the reproductive tract, and so on. The bodilyvessel can be natural, or a graft, such as an autologous graft or asynthetic graft. Two further exemplary embodiments of applications otherthan to AV grafts will now be described.

(A) Hypoplastic Left (or Right) Heart Syndrome

In this condition, blood is supplied by only a single ventricle of theheart. Referring to FIG. 13, pulmonary circulation must often be assuredby providing a shunt 200 connecting the subclavian or innominate artery202 to the right or left pulmonary artery 204. This is also known as themodified Blalock-Taussig shunt. The shunt 200 itself is a vasculargraft, such as a PTFE tube. Survival of the patent is often dependent onthe optimal distribution of flow between the shunt 200 and the aorta206.

According to the present invention, a variable flow control device 20 isplaced on the shunt 200. The shunt 200 drives flow from the systemiccirculation in the innominate artery 202 to the pulmonary circulation inthe pulmonary artery 204. The variable flow control device 20 is, forexample, according to any one of the above described devices. The flowcontrol device 20 enables the flow in the shunt 200 to be regulated andaccording to a method of the invention, the flow in the shunt 200 iscontrolled to equilibrate the repartition of flow between the systemicand pulmonary circulation.

(B) Esophageal Banding or Replacement of the Esophagus Valve

The valve at the end of the esophagus connecting the esophageal tube tothe stomach may fail, causing re-entry of food from the stomach to theesophagus and consequent discomfort to the patient. Also, for thetreatment of obesity, sometimes a banding at the end of the esophagusmay be surgically placed. The banding causes a localized restriction tothe esophageal tube. Banding is not a precise procedure and is notadjustable without further abdominal surgery. According to the presentinvention, a variable flow control device, such as embodied above, islocated on the esophagus to alleviate either of these problems. Thedegree of esophageal restriction can be easily controlled telemetricallyto allow controlled passage of food into the stomach when required butto restrict it at other times or to prevent re-entry of food from thestomach into the esophagus.

Whilst specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

What is claimed is:
 1. A constriction device for adjustably controllinga flow in a bodily vessel of a patient, the device comprising: anenclosure for implantation into the patient, the enclosure defining acavity therein; a flexible membrane positioned within the cavity of theenclosure and configured to constrict the bodily vessel; a heatdeformable element positioned within the cavity of the enclosure andhaving a normal state and a deformed state, the heat deformable elementconfigured to change from the normal state to the deformed state whenheat is applied to the heat deformable element; and a moveable memberpositioned within the cavity of the enclosure and coupled to theflexible membrane and the heat deformable element, the moveable memberconfigured to apply a force on the flexible membrane based upon the heatdeformable element being in the normal state or the deformed state. 2.The device of claim 1 wherein the heat deformable element is configuredto change from the deformed state to the normal state if the heat is notapplied to the heat deformable element.
 3. The device of claim 1 furthercomprising an implantable heater positioned adjacent to the heatdeformable element for applying the heat to the heat deformable element.4. The device of claim 3 wherein the implantable heater comprises anantenna that heats when an appropriate electromagnetic field is appliedto the antenna.
 5. The device of claim 3 wherein the implantable heatercomprises an electrical resistance.
 6. The device of claim 5 furthercomprising an implantable battery, the battery configured to supply anelectric current to the implantable heater.
 7. The device of claim 3further comprising a radio frequency converter, the converter configuredto convert radio frequency radiation into electric power for poweringthe implantable heater.
 8. The device of claim 1 further comprising anextracorporeal heat source configured to be directed onto the patientnear the heat deformable element for applying the heat to the heatdeformable element.
 9. The device of claim 8 wherein the heat sourcecomprises an infrared lamp.